1. Field of the Invention
This invention relates to integrated circuit inductors and more particularly to integrated circuit inductors having a high self-resonance frequency and a high quality factor, Q.sub.n.
2. Description of Related Art
It is desirable to include inductors on integrated circuits ("IC") versus off the IC to reduce the form factor and cost of devices requiring inductors. IC inductors, however, commonly have unacceptably low quality factors, low self-resonance, and cause interference with surrounding or neighboring IC components. This is particularly true for IC inductors used in high frequency applications such as in the front end of radio frequency ("RF") receivers and transmitters.
Due to the significant advantages of having IC inductors, several solutions have been investigated. First, inductors having different geometric patterns such as those shown in FIGS. 1A to 1C have been analyzed including a spiral inductor 42, a simple loop inductor 44, and a meander inductor 46. Of these inductors, investigation has revealed that spiral inductors are most easily adapted for inclusion in silicon ICs because a desired inductance can be achieved using a smaller surface area with a spiral inductor versus a loop or a meander inductor.
As way of background FIG. 2A shows such an inductor 50 which generates magnetic field lines 52 due to the flow of current shown in the FIGURE. FIG. 2B illustrates a cross sectional view of an IC structure 54 including the inductor 50. The IC 54 includes a silicon substrate 56, a conductive ground plane 58, and a silicon oxide layer 60. When the inductor 50 is included on an IC such as shown in FIG. 2B and current flows through the inductor 50 such as shown in FIG. 2A, the lines of magnetic field can enter the substrate 56 and be significantly reduced. This reduces the quality factor (Q.sub.n) of the inductor. In addition the frequency at which the inductor 50 self-resonances is also reduced due to parasitic capacitance between the inductor 50 and the substrate 56.
In order to limit the passage of the magnetic fields of an inductor into the IC's substrate, insulating regions and ground plates have been inserted between the inductor and substrate. For example, U.S. Pat. No. 5,539,241 to Abidi et al. teaches etching a pit under the inductor to create an insulating region between the inductor and the substrate. As noted in the PCT application number US98/05149 to Yue et al. (which is discussed below), the inclusion of this insulating region may effect the mechanical integrity of the IC, in particular, the inductor. In addition, the construction of an IC with such insulating region is likely to be expensive and complex.
Yue et al. also discusses another IC inductor structure where the structure includes a solid metal conducting ground shield 72 such as shown in FIG. 3A. As shown in FIG. 3B, the solid conducting ground shield 72 is placed between the inductor 50 and the substrate 56. As shown in FIG. 3A, an image current 76 is induced from the magnetic field lines generated by the inductor 50. This induced image current 76 generates magnetic field lines 74 where the flux of the magnetic field is opposite the flux of the magnetic field of the inductor 50. Consequently, while this IC inductor configuration isolates the inductor 50 from the substrate 56 (no magnetic coupling), the configuration substantially reduces the effective inductance of the inductor and thereby the Q.sub.n of the inductor. In order to overcome these problems, Yue et al. suggests an IC inductor configuration that includes a patterned ground shield 80 between the inductor 50 and substrate 56 as shown in FIGS. 4A and 4B. The patterned ground is a polysilicon ground shield that includes locally isolated conductive lines 84 separated by slots 85 where the conductive lines 84 are orthogonal to the conductive line segments of the inductor 50. Yue et al. teaches that the orthogonal relationship between the conductive lines 84 and conductive line segments reduces or eliminates the generation of an image current in the ground shield 80. The ground shield effectively terminates any electric field generated by the inductor 50 thus limiting any leakage into the substrate 56. The termination of the electric field at the ground shield 80, however, may effectively generate a quasi capacitor where the inductor acts as one plate and the ground shield acts as another plate. The relative proximity of the inductor to the ground shield increases its effective capacitance. Consequently, the IC inductor configuration of Yue et al. may not have an acceptable quality factor at high frequencies.
It is also noted that Yue et al. suggests that the substrate 56 is a low resistivity substrate. In particular, Yue et al. includes examples of an 11 ohm-cm silicon substrate and a 19 ohm-cm silicon substrate. It has been found that low resistivity substrates employed in IC inductor configurations may also lower the self-resonance frequency of the inductor regardless of the ground shield or grounding technique employed. Thus, a need exists for an IC inductor configuration that has a high self-resonance frequency and an acceptable quality factor at high frequencies.